Various systems have been proposed to alert a driver to the presence of an electromagnetic signal such as that in police radar.
Decker et al., U.S. Pat. No. 5,164,729, is directed to a police radar detector which includes an auto-mute feature by which a driver may be alerted to the presence of police radar but without an unnecessarily prolonged audible alarm. Upon the detection of a police radar signal, an audible alarm begins to beep at a selected intensity and at a beep rate corresponding to the strength of the signal. At a time after detection of the police radar signal, the intensity of the audible alarm is automatically reduced.
Grimsley, U.S. Pat. No. 4,631,542, is directed to a radar warning receiver having an alarm indicator responsive to an electronic switch, the switch being activated and energizing the alarm in response to an alarm signal indicating the presence of a police radar signal, the switch being deactivated and de-energizing the alarm in response to a mute signal occurring during the occurrence of the alarm signal, the switch remaining deactivated until the alarm condition signal terminates and subsequently reoccurs.